Richard Burdon Haldane, 1st Viscount Haldane in the context of Haldane Mission

The British Expeditionary Force (BEF) was the formation of the British Army on the Western Front during the First World War. In its original form it comprised six divisions, sent by Britain to France in 1914 to aid in resisting the German invasion.

Planning for a British Expeditionary Force began with the 1906–1912 Haldane Reforms of the British Army carried out by the Secretary of State for War Richard Haldane following the Second Boer War (1899–1902).

The Haldane Reforms were a series of far-ranging reforms of the British Army made from 1906 to 1912, and named after the Secretary of State for War, Richard Burdon Haldane. They were the first major reforms since the "Childers Reforms" of the early 1880s, and were made in the light of lessons newly learned in the Second Boer War.

The major element of the reforms was the creation of an expeditionary force, specifically prepared and trained for intervening in a major war. This had existed before, but it had not been well-prepared for overseas service; the newly organised force was to have a permanent peacetime organisation and a full complement of supporting troops. At the same time, the reserve forces were restructured and expanded so as to ensure that overseas forces could be expanded, supplied with new recruits and to provide for home defence. The Volunteer Force and the Yeomanry were reorganised into a new Territorial Force and the Militia was formed into the Special Reserve; these latter two reforms were grouped together in the Territorial and Reserve Forces Act 1907. To encourage the development of military skills, an Officer Training Corps was established in public schools and universities. Taken together these measures were designed to bring the Army and wider public closer together, to deliver Haldane's vision of a 'nation in arms'. Military strategy was revitalised by a new Imperial General Staff, which would ensure a common doctrine and common strategic aims among the various military forces of the British Empire, including the Dominions as well as British India. Finally, the Regular Army itself would be reformed by the development of a new operational and training doctrine, laid down in Douglas Haig's new Field Service Pocket Book.